Some bricks, such as the Erlingbrick, seem to have common nicknames in the LEGO building community. As the basic 2 x 4 brick appears to be the foundation of the system, it is reasonable to suppose that it would have such a nickname.

3 Answers
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This may sound like a weird or mocking answer, but when you think about it, it does make sense.

The 2x4 brick the most iconic brick of them all, the one which has started it all. The one for which the first patent has been issued in 1958. The book "50 Years of the LEGO brick" contains 6 of them, and the number of permutation for these 6 bricks has been the classic question to illustrate how endless LEGO construction can be (even LEGO got the answer wrong at first, by the way).

An Erling brick is actually a piece, or a component. Considering the first LEGO sets designed were based purely on bricks, we've adopted the usage of the word brick for use on other LEGO elements.

Of all the LEGO 'bricks', the 2x4 seems to have the most resemblance with a typical brick used in constructing real life buildings. When people mention a LEGO brick, most people tend to imagine the 2x4 brick.